1. Field of the Invention
The present invention relates to a photovoltaic device and a method of manufacturing thereof. More specifically, the present invention relates to a so-called series type photovoltaic device wherein a plurality of series connected photoelectric conversion cells composed of semiconductor layer such as amorphous silicon are arranged along the length of a single substrate, and a method of manufacturing thereof.
2. Description of the Prior Art
This kind of photovoltaic device is disclosed, for example, in U.S. Pat. No. 4,281,208, assigned to the same assignee as the present invention. A brief description will be given here of the structure of this photovoltaic device shown in FIG. 1 within the context required for understanding the present invention.
A plurality of photoelectric conversion cells 2a 2b, 2c,--are formed on a glass substrate 1. Transparent electrodes 3a, 3b, 3c, -- are formed with a constant interval between adjacent photoelectric conversion cells 3a, 3b, 3c, --. On the respective transparent electrodes 3a, 3b, 3c, --, semiconductor photo-active layers 4a, 4b, 4c, -- are formed, which are composed of amorphous silicon or the like. On the semiconductor photo-active layers 4a, 4b, 4c, --, back electrodes 5a, 5b, 5c, -- are formed, the ends of which extend to the adjacent transparent electrodes 3b, 3c, -- to be connected thereto.
The semiconductor photo-active layers 4a, 4b, 4c, -- comprise, for example, PIN junctions parallel with the film surfaces, and when the light enters into them through the glass substrate 1 and the transparent electrodes 3a, 3b, 3c, -- photovoltages are generated in the respective semiconductor photo-active layers 4a, 4b, 4c, --. The photovoltages generated in the respective photoelectric conversion cells 2a, 2b, 2c, -- are added in series because the back electrodes 5a, 5b, 5c, -- are connected to the adjacent transparent electrodes 3b, 3c, --.
Normally, in order to manufacture a photovoltaic device of such a structure, a photo-etching technique is employed for its micro-workability. In the case of employing the photo-etching technique, with reference to the example shown in FIG. 1, a transparent electrode layer is formed on the whole of one main surface of the glass substrate 1, and photo-resist films are formed on the parts corresponding to the semiconductor photoactive layers 4a, 4b, 4c, --, and then etching is performed and thereafter the photo-resist films are removed, and thereby the semiconductor photo-active layers 4a, 4b, 4c, -- for the respective photoelectric conversion cells 2a, 2b, 2c, -- are formed.
Such a photo-etching technique excels in microworkability, but is likely to produce defects in the semiconductor photo-active layer due to pinholes produced in the photo-resist film defining the etching pattern, peeling-off at the fringe of the photo-resist film, etc..
Subsequently, a method not employing photo-etching techniques was proposed,for example, in U.S. Pat. No. 4,292,092 issued on Sept. 29, 1981. In this Patent, a laser beam is employed. This method which performs patterning by irradiating the laser beam is extremely effective in that micro-working can be performed without employing any wet processing.
However, conventional working by means of laser irradiation has the following problems to be solved. Specifically, the working by the laser beam is essentially a thermal working, and therefore if another layer is present under the part of layer to be worked, it is important not to damage it. Otherwise, in addition to burning-off the desired part of the layer, the under layer not required to be burnt-off is also burnt-off, or if not so, it thermally damaged. In U.S. Pat. No. 4,292,092 as cited above, in order to meet this requirement, it is proposed that the laser output or the pulse frequency is selected specifically for each film or layer to be worked.
However, even by this prior art method, the workability is still insufficient because of variations of the film thickness of the semiconductor photo-active layer which are inevitably present. Specifically, the absorption factor of the laser beam varies greatly depending upon the thickness of the film or layer to be worked, and therefore the threshold energy density of the laser for scribing is not always constant. For example, in the case of amorphous silicon, the relationships of absorption factor A, reflection factor R, and transmission factor T of the laser beam to the film thickness are as shown in FIG. 2. As is obvious from FIG. 2, for example, in the case of working amorphous silicon films by a Nd:YAG laser of 1.06 .mu.m wavelength with Q switching, the absorption factor of the laser radiation varies greatly within a range of 5%--20% at a film thickness of 4000 .ANG. or more which is practicable for the photovoltaic device. Accordingly, in the case of working amorphous silicon films by such a YAG laser, when a high laser output is used so as to scribe effectively even if the film thickness is such to give a minimum absorption factor of 5%, a laser beam having an output of four times the threshold energy density is irradiated onto those parts of the film having a thickness corresponding to a maximum absorption factor of 20%. Accordingly, thermal damage to the transparent electrode present under such parts of the amorphous silicon film is unavoidable. Conversely, when a low laser output is employed so that those parts of the film having a thickness corresponding to an absorption factor of 20% can be worked, the laser energy is insufficient at those parts of the film having a thickness corresponding to an absorption factor of 5%. Accordingly, amorphous silicon at those parts is not removed, and remains uncut, resulting in a reduction in output of the photoelectric conversion cell.
Thus, one problem in U.S. Pat. No. 4,292,092 is that since the absorption factor of the laser beam varies greatly depending upon the film thickness of the amorphous silicon film, partial thermal damage is given to the under-layered transparent electrode or the amorphous silicon film at those parts which remain uncut.
U.S. Pat. No. 4,292,092 has another problem as follows: In general, a metal film having a high heat conductivity such as an aluminum, silver or the like is employed for the back electrode in such a photovoltaic device. In the case where the laser beam is irradiated onto such a back electrode of aluminum, various disadvantages described in the following are caused since the working conditions are stringent.
For example, as shown in FIG. 3, in the case of a structure wherein the back electrode on the transparent electrode exposed by the semiconductor photo-active layer is separated, for example, the back electrode 5b on the semiconductor photo-active layer 4b is melted due to heating by a laser beam of large output, and a melted part 5ab flows onto the transparent electrode 3b, causing the photoelectric conversion cell 2b to short-circuit. Also, as shown in FIG. 4, in the case of a structure wherein the back electrode on the underlying semiconductor photo-active layer is separated, the parts of the semiconductor photo-active layer 4b bombarded directly by the laser beam of large output are annealed, and the resistance at those parts 4b' is lowered. Consequently, the back electrode 5a and 5b which may be separated physically from each other are not separated electrically because of the low resistance of the part 4b' of the semiconductor photo-active layer 4b', and accordingly, the open-circuit voltage Vcc of the whole photovoltaic device is reduced.
Another laser-beam technique capable of solving one of the problems of U.S. Pat. No. 4,292,092 is disclosed, for example, in U.S. Pat. No. 4,517,403 issued on May 14, 1985. In this Patent, the back electrode of each photoelectric conversion cell is connected in series to the adjacent transparent electrode through silver paste buried in the amorphous silicon. In this Patent, the amorphous silicon layer is not required to be scribed, and therefore the first problem of U.S. Pat. No. 4,292,092 is avoided, namely the problem caused by variation of the laser beam absorption factor due to variation of the film thickness of the amorphous silicon layers. However, this Patent still does not solve the second problem of U.S. Pat. No. 4,292,092, namely scribing of the back electrode.
Also, in U.S. Pat. No. 4,668,840 issued on May 6, 1987, it is proposed to insert a heat insulating material between the back electrode and the semiconductor photo-active layer in order to remove the deleterious heating effect due to laser-scribing of the back electrode. This U.S. Pat. No. 4,668,840, is characterized in that no damage is given to the underlying semiconductor photo-active layer or the like even when a laser beam of a relatively large output is employed in scribing the back electrode. However, U.S. Pat. No. 4,668,840 gives no consideration to the change in absorption factor due to the variation of film thickness because the semiconductor photo-active layer itself has been already scribed in the previous process, still leaving the first problem of U.S. Pat. No. 4,292,092.
On the other hand, aforementioned transparent electrodes 3a, 3b, 3c, -- are normally composed of transparent conductive oxide(TCO) typified by tin oxide(Si0.sub.2), indium oxide(In.sub.2 O.sub.3) or indium tin oxide(ITO). The sheet resistance of such a TCO is approximately 30-50 ohms/cm.sup.2 which is more than three-times that of a metal such as aluminum, gold, silver or the like, and therefore, it is known that an electric power loss is caused in the transparent electrodes 3a, 3b, 3c, --. In order to reduce such an electric power loss in the transparent electrodes 3a, 3b, 3c, --, the assignee of the present invention has proposed a structure in which a plurality of strip collecting conductors having good electric conductivity are formed on a whole area of the transparent electrodes composed of TCO in, for example, Japanese Patent Laying-Open No. 130977/1981 laid open on Oct. 14, 1981.
In such a proposed structure, the composite sheet resistance of the collecting conductors and the transparent electrodes becomes small and thus the electric power loss is reduced; however, since the collecting conductors are laminated on the transparent electrode the bottom surface of the semiconductor photo-active layer becomes uneven and thickness of the portions of the semiconductor photo-active layer above the collecting conductors become thin, and therefore, accidental short-circuits occur when the collecting conductors penetrate the semiconductor photo-active layer and then contact the back electrode.
Therefore, the assignee of the present invention has further proposed to arrange an insulator film on the collecting conductors for preventing the above described short-circuit accident in, for example, Japanese Patent Laying-Open No. 125668/1984 laid open on July 20, 1984.
However, in such a structure in which the insulator film is arranged on the collecting conductors, if the thickness of the collecting conductors is made large to further reduce the above described composite sheet resistance, the thickness of the collecting conductors penetrating the semiconductor photo-active layer becomes large and thus the side surfaces of the collecting conductors are exposed, and consequently the side surfaces of the collecting conductors come into contact with the back electrode. This means that accidental short-circuits cannot be completely prevented even with such a structure. Also, even if accidental shortcircuits are prevented, since the side surfaces of the collecting conductors directly contact the semiconductor photo-active layer, metal of the conductors, such as aluminum, silver or the like is diffused into the semiconductor photo-active layer, and subsequently a deterioration of a film quality of the semiconductor photo-active layer takes place.